Thread rolling attachment



Jan. 5, 1960 H. L. BERKEY ET L THREAD ROLLING ATTACHMENT 2 Sheets-Sheet 1 Filed April 29, 1957 INYENTOR HARRY L. BERKEY FREDERIC NELL ATTORNEYS Jan. 5, 1960 H. L. BERKEY E ,9

THREAD ROLLING ATTACHMENT Filed April 29, 1957 2 Sheets-Sheet 2 1 5'- 5 INVENTOR HARRY L. BERKEY FREDERIC NELL ATTORNEYS United States PatentO THREAD ROLLING ATTACHMENT Harry Law Berkey and Frederic Nell, Waynesboro, Pai, assignors to Landis Machine Company, Waynesboro, Pa., a corporation of Pennsylvania Application April 29, 1957, Serial No. 656,480

8 Claims. (Cl. 80-6) This invention relates to tools or appliances for rolling screw threads and particularly to appliances for automatic lathes or the like wherein a pair of spaced threading rolls are mounted in a holder and are fed laterally by the holder toward a rotating work piece.

In a device of this kind, the path of movement of the thread forming serrations on each roll is substantially tangential to opposite sides of the work piece. Thus the initial contact between rolls and work piece takes place in a plane spaced laterally from a plane containing the axes of the rolls. Consequently, the thread on the work piece is started at two points on the periphery thereof which are not diametrically opposite and the thread rolling operation is completed at two points on the periphery of the work piece which are diametrically opposite. It will be seen that the axial relationship of the thread forming rolls must be different at the start and finish of the operation. This condition, if not compensated for, produces irregular helices on the work piece and unduly stresses the thread forming serrations of the rolls.

This consideration has led to a number of prior constructions. In one case, one of the rolls is permitted to travel at a rotational speed slightly different from that of the other roll in order that the axial relationship of the threading rolls may vary to conform to the varying angle ofcontact with the work piece. In another case, the rolls are constrained to rotate at the same speed and one of the rolls is held axially stationary while the other is permitted to slide in an axial direction to maintain the axial or tracking relationship of the two rolls. In both cases the relationship between diameters of rolls and work is such that the axially stationary or definitive roll does not form a true helix on the work piece at the start of the operation and therefore is subjected to considerable endwise pressure throughout the operation. The result is excessive wear throughout the entire tool and particularly on the threads of the stationary roll, causing frequent replacement of the expensive rolls and defective threads. The prior devices have not solved the problems of correctly relating the diameters and positions of the threading rolls to the diameter and position of the work piece.

.It has been discovered that the above disadvantages of the prior constructions can be obviated and a true helical rolling action without axial pressures can be achieved by taking the lateral movement of the tool relative to the work into account when the diameters of the rolls are determined. This lateral movement has the effect of augmenting the rotational movement of one roll and reducing the rotation of the other. According to the present invention, a differential between the diameters of the rolls is established such that the threads of the rolls are in substantially perfect registry with the threads of the work piece throughout the threading operation. It has also been discovered that this difference in diameters requires that the .rolls have different helix 2 The different helix angles of the rolls will also differ mutually from the helix angle of the work piece. According to a further feature of the invention, it has been discovered to be preferable to incline the axes of both rolls relative to the axis of the work piece, to the angle necessary to align the respective helix angles of the rolls with the helix angle of the work piece and thus eliminate the posibility of any axial pressures from this source. Under these conditions the total axial lead of the thread on both rolls will be the same and will coincide at all points with the lead of the thread on the work piece.

Thus, if the threading rolls are positively connected as, for example, by gears to move at the same angular velocity no strain is imposed upon the gears during the thread rolling operation if the thread rolling mechanism is constructed in accordance, with the present invention. At the termination of the feeding movement of the tool onto the work piece, the tool dwells and then is withdrawn laterally at a speed which is usually substantially greater than the rate of the feeding movement. Since the rolls are compensated for lateral feeding movement of the tool during the threading operation, the dwell and subsequent withdrawal movement in the opposite direction at a much higher speed imposes a considerable stress on the teeth of the connecting gears. If the dwell period is excessive the work may be objectionably marked. These disadvantages are eliminated according to a further aspect of the invention in accordance with which the gears and rolls are resiliently connected so that the reversal stresses occasioned by the dwell and lateral withdrawal of the tool are absorbed without damage to the gears or work piece.

The present invention comprises a structure embodying the above principles in a particularly efiicient, rigid and accurate manner. The inclination of the roll axes for helix angle alignment is accurately accomplished by means of members which may be replaced for various degrees of inclination and which permit inclination in no plane other than the one desired. The use of rolls of various diameters and operations upon work pieces of various sizes is made possible by providing a two-part holder for the rolls which may be adjusted to any size within the range of the tool.

Accordingly, it is an object of the invention to provide, a two-roll thread rolling mechanism for rolling a thread while moving laterally toward the axis of a work piece in which the two rolls are of different diameters to com-- pensate for the lateral movement of the mechanism.

It is a further object to provide a mechanism of the class described in which, in a threading operation, the number of revolutions of the work piece is a whole number multiple of the number of rotations of each roll.

It is another object to provide such a mechanism in which lateral pressures or forces exerted upon the rolls during thev rolling operation are reduced to a value here-. tofore unobtainable.

A still further object of the invention is the provision. of means for aligning the threads of each roll with the threads of the work and the provision of means for in-: clining the axes of the rolls in a direction to accomplish such alignment to various degrees without inclination in any other direction.

An additional object is to provide a novel resilient driving connection between the rolls which is etfective: only during the period following the threading operations A further object is to provide a novel resilient connection between the rolls which is effective to permit relative angular movement between the rolls when they are withdrawn from the work and to thereafter auto-: matically restore the rolls to proper Fis i g relationship. for a succeeding threading operation.

Another object is to provide a mechanism of the above type which is extremely rigid and capable of maintaining accuracy for a long period of time and at the same time adjustable for various sizes of work pieces and threading rolls.

Other objects and advantages of the invention will be apparent from the following description and the accompanying drawings which show one embodiment thereof. In the drawings Figure 1 is a side elevation of an assembled mechanism embodying the present invention;

Figure 2 is a vertical section along line 22 of Figure 1;

Figure 3 is a vertical sectional view along line 33 of Figure 2;

Figure 4 is an elevation of the side opposite to that shown in Figure l, with the shank for mounting the device shown in section;

Figure 5 is a vertical sectional view along line 5-5 of Figure 4;

Figure 6 is a partial sectional view along line 6-6 of Figure 1;

Figure 7 is a fragmentary sectional view along line 7-7 of Figure 1;

Figure 8 is a fragmentary sectional view along line 8-8 of Figure 5;

Figure 9 is a fragmentary sectional view along line 9--9 of Figure 5;

Figure 10 is a vertical sectional view along line 10-10 of Figure 5;

Figure 11 is a fragmentary sectional view along line 1111 of Figure 3;

Figure 12 is a diagrammatic view illustrating the operation of a thread rolling mechanism having rolls of equal diameter;

Figure 13 is an end view of a modified roll driving gear;

Figure 14 is a vertical sectional view taken along line 14-14 of Figure 13; and

Figure 15 is a diagrammatic illustration of the relative disposition of the roll axes, the angular relation being exaggerated for illustrative purpose.

As pointed out above it has been discovered that the use of threading rolls of equal diameters leads to inaccuracies in operation of a device of this type and imposes considerable axially-directed stress upon the threads of the rolls and consequently reduces the active life of the rolls and leads to the production of threads having inaccurate helices. In Figure 12 are shown, diagrammatically, two threading rolls and 22, a work blank W at the start of the threading operation and a finished work piece W at the end of the threading operation. It will be understood that the rolls will rotate at the same speed, in the same direction and opposite to the direction of rotation of the work piece. At the same time, both rolls move rectilinearly as a unit toward the work piece as the threading operation proceeds.

In order that a full understanding of the invention may be had, the theoretical action of the thread rolls and the work piece will be considered in detail to demonstrate mathematically quantitatively and qualitatively the error produced by the rolls of the prior art.

It will be assumed initially for purposes of this discussion that rolls 20 and 22 are of equal diameter D in accordance with prior practice. In accordance with established principles the diameter D is a whole number multiple of the blank diameter d of the work piece. The rolls 20 and 22 make initial contact with the work blank at points a and b respectively on the respective major diameters of the rolls and the blank diameter of the work piece. The points a and b are points of tangency of the work blank W and the rolls and lie on the lines x and y, respectively, joining the center of the work blank with the respective roll centers, said lines making equal angles A and A, respectively, with the line joining the centers of the two rolls. Considering a as two points, one a on the roll 20 and one a on the work blank, the thread on the roll 20 and the embryo thread on the work piece will be in registry initially at the points 61 and a At the finish of the operation the respective rolls 20 and 22 will be in contact with the work piece at the points 0 and e, 180 apart on the work piece. Points on the work blank 180 apart on a line parallel to the diameter containing points 0 and e are designated by f and g respectively. The line z connecting the points 1 and g makes equal angles B and B with the respective lines x and y, the angles B and B being equal to the angles A and A.

The point (1 on the major circumference of the roll 20 will travel during the rolling operation a distance equal to N times the roll circumference or N1rD, where N equals the predetermined number of revolutions made by the roll during the rolling operation and D equals the major diameter of the roll. Assuming N to be a whole number, the point will complete its travel at the same angular location at which it started, that is, on the line x.

Now if n is the number of revolutions made by the work piece during the rolling operation 11 equals which is a whole number, the point a on the circumference of the work blank theoretically will travel during the rolling operation a distance equal to n times the blank circumference or mrd which is equal to N1rD. Thus point a on the work piece theoretically will also com plete its travel at the same angular location at which it started, that is, on the line x. However, since the work piece has moved bodily to the position W during. the rolling operation, the line x has assumed the new} position x. Thus while the point a on the roll 21) has returned to its starting position, the point a on the work piece has actually arrived at the line x. In other words the point a on the roll 20 has passed the point of de sired registry c and has therefore exceeded its desired travel by an amount equal to the length of the are ac.

At the same time the point a on the work piece has failed to reach the desired point of registry c and has therefore fallen short of a desired extent of travel by an, amount equal to the length of the arc af. Thus registry. between roll 20 and the work piece is lacking by a distance equal to are are are a).

Similarly the point b fails to reach the point of registry e and therefore falls short of its desired extent of travel by an amount equal to the length of the are be. At the same time the point b on the Work piece passes the point of registry and therefore exceeds its desired travel by an amount equal to the length of the arc bg. Thus registry between roll 22 and the work piece is lacking by a distance equal to are be plus bg which is also equal to are ac plus arc af.

In practice this lack of registry between the rolls and the work piece will produce severe distortion of the threads on the work piece and objectionably high axial loads on the threads of the forming rolls.

While insofar as is known the above analysis has not been made, this problem has long been recognized and it has been proposed to eliminate these objectionable results.

either by permitting relative axial movement of the rolls or relative rotary movement of the rolls. However, neither of these proposals has been fully satisfactory and they do not result in the production of perfect threads nor the elimination of forces which tend to produce rapid wear of the forming rolls. In accordance with the present invention it has been discovered that this problem can be solved without either of the prior expedients solely by' cumference.

piece both to increase and to decrease. It has been discovered that this correction, which is not possible, can also be taken care of by correction of the roll diameters.

Specifically if the travel of point a on the roll 20 is diminished by the length of the are ac and by the length of the arc af the roll 20 and the work piece will be in registry at the point c. Similarly if the travel of point b on the roll 22 is increased by the length of the are be and by the length of the arc bg the roll 22 and the work will be in registry at the point 2. Since the are be equals the are ac and the arc bg equals the are a the expression (arc ac-l-arc af) is the roll size correction factor for N revolutions of the rolls in terms of cir- The correction factor in terms of circumference for one revolution of the rolls is (are ac+arc af) divided by N. The correction factor in terms of diameter for one revolution of the rolls is (are ac-l-are af Nrr In other words the correction factor is the quotient resulting from dividing pi into the distance of the peripheral movement of the point of contact between one of the rolls and the work piece in one revolution of said roll.

The diameter D referred to above is customarily calculated by using as a basis a diameter of work piece halfway between the blank diameter (which is customarily equal to the pitch diameter of the thread to be rolled) and the root diameter of the finished thread. Thus if d equals the work blank diameter, h equals the depth of the thread to be rolled, and S is the ratio of the lead of the thread on the rolls to the pitch of the thread to be rolled, the roll diameter D may be expressed by the in which S is a whole number and the expression are ac-l-arc of Also, the optimum or corrected major diameter D of roll 22 is equal to the normal diameter D plus the correction factor in terms of diameter, that is:

are ac-l-arc af N1r The above discussion has been predicated upon the assumption that the rolls 20 and 22 are of the same diameter. It can be demonstrated mathematically that the error introduced by this assumption is so slight that it is beyond the limits of machining tolerances and is thus of no practical consequence.

The practical application of the above formulae may be illustrated by reference to a specific example. Assuming that it is desired to compute the proper roll diameters in accordance with the principles of the present invention for rolling 20 threads per inch NF on a Z5 inch work piece with a roll having four starts and that the rate of penetration is such that the work piece makes 25.5 turns during the rolling operation, the following calculations may be made. N =6.375 turns. The basic pitch diameter of the finished work piece is .405", the blank diameter is .404" and the basic thread depth is .0325". The application of the formula given above for determining the normal roll diameter to these figures yields a Substituting the numerical values the correction factor is .00892. The corrected diameter of roll 20 D equals D minus the correction factor (1.551" minus .0089") or 1.5421". The diameter D of roll 22 equals D plus the correction factor (1.551" plus 0.0089") or 1.5599. Thus it will be apparent that the corrected roll diameters are derived from the normal roll diameters by applying a correction factor to the latter which in each case is equal to the quotient obtained by dividing pi into the distance of the peripheral movement of the point of contact between one of said rolls and the work piece in one revolution of said roll. In other words the correction factor is the peripheral distance traveled during the threading operation by a point of contact between a roll and the work piece divided by the expression pi N.

In this specific example the portion of the correction factor attributable to are B is approximately .00133. Practical experience has demonstrated that this factor can be ignored except for work of the highest quality. Thus for forming threads on work pieces to most presently existing standards the formula for the correction factor in terms of diameter may be considered to be Arc A Nrr It has been discovered that optimum rolling conditions exist when the mating threads of roll and work piece are inclined in the same direction. Therefore, according to the present invention, the axes of rolls 20 and 22, which are normally parallel, are inclined from this normal position in parallel planes containing the roll axis and normal to a line joining corresponding points on the roll axis as illustrated diagrammatically and on exaggerated scale in Figure 15.

Since both rolls necessarily have the same number of thread starts, the helix angles of the two rolls will differ from each other. The helix angles of both rolls will be slightly larger than the work piece helix angle since the reduction in roll diameter in determining the normal diameter is greater than the increase in the diameter of roll 22 to determine the corrected diameter. The amount of the inclination of the roll axis for a given roll is equal to the diflerence between the roll helix angle, based on the corrected major diameter thereof, (k or k and the work piece helix angle, k, based on the pitch diameter of the work piece. The direction of such inclination is such that respective contacting portions of the thread ridges of both rolls and work piece extend in substantially parallel relationship at the start of the rolling operation.

A specific example will be given by way of illustration: with reference to the specific work piece and rolls referred to above. If k is the helix angle of the work piece,

7 is the helix .angle of the roll 20 based on the corrected major diameter,

thus equals 2 21 50". Accordingly, the inclination of the axis of roll 28 is 6'28". If k is the helix angle of roll 22 based on the corrected major diameter,

thus equals 2 20 11" and the inclination of the axis of roll 22 is 4' 49".

As shown in the remaining drawings, a structure embodying the above principles comprises a holder member 24 adapted to be attached to the cross tool slide of an automatic screw machine or the like for reciprocal movement in a direction perpendicular to the axis of a work piece held in and rotated by a chuck (not shown) of the machine, as well known in the art. The holder 24 is formed with two flat, forwardly projecting arms 26, 28 (Figures 1, 2, 3, 4 and 6) which are connected near their forward extremities by a shaft 30. A flange 32 is provided at one end of shaft 30, outwardly of the arm 26. A screw 34, threaded into the side of arm 26, overlaps a portion of the flange 32 to retain the shaft 30 axially in position. A knurled extension 36 may be added to shaft 3h, outwardly of the flange 32, for manual removal of the shaft 30, after removal of the screw 34.

Upon a bushing 38 (Figure 6) journalled upon shaft 30 between the arms 26 and 28, upper and lower frame members 46 and 42, respectively, are pivotally mounted. The upper frame member 48 is provided with spaced depending portions 44 which lie adjacent the opposite sides of an upward extension 46 of lower frame member 42, the bushing 38 passing through portions and extension 46. The ends of the bushing 38 should be flared outwardly after assembly of the members 48 and 42 to prevent relative axial movement between the frame members and the bushing.

Aligned recesses 48, of rectangular cross station are formed in the rear surfaces of frame members 40 and 42 to receive strap member 54? (Figures 2 and 3). Screws 52 and 54 secure the strap 59 to frame members 40 and 42 respectively and effectively prevent relative rotation of the frame members about shaft 30 while the device is in operation. The opening in strap 50 for screw 54 is slightly elongated to permit the strap to move relatively to screw 54. It will be understood that the strap 50 and associated parts are effective only to prevent relative movement between the frame members 40 and 42 which may pivot as a unit about shaft 36 as is conventional in the art. If desired, the frame members may be spring biased to a desired position as shown in U.S. Patent 2,694,322. A suitable spring is shown at 55 in Figure 1.

An adjustment for determining the spacing of the rolls will now be described. A threaded opening 56 (Figures 1, 11), extending inwardly from the side of frame member 42, is provided to receive a plug 53 having a beveled surface 6% on its inner end, and a set screw 62, by means of which the plug is held in adjusted position. Another opening 64, perpendicular to and communicating with opening 56 and also communicating with an end of the recess 48, receives a plug 66 having a bevel 68 on its inner end in contact with the bevel -60 on the plug 58. A second bevel 7%) (Figure 3) on the outer end of plug 66 makes contact with one end of strap member 58. Thus, when the screw 54 is loosened, screw 62 may be rotated to move the plugs 58 and 66 axially and thereby effect a fine adjustment of the relative rotative position of frame members 4-0 and 42.

As seen in Figure 3, perpendicular openings 72 and 74 similar to openings 56 and 64, respectively, are formed in the top frame member 40 to accommodate plugs 58 and 66 and screw 62 when the circumstances of use render the opening 56 inaccessible, as the opening 72 is in the tan I620 kgo tan kgg:

present instance. In such case, the strap 50 is reversed, end for end, to accomplish the above-described adjustment. For a major adjustment of the relative rotative position of the frame members 40 and 42, a different strap member of different length is substituted for strap member 50.

A relatively wide slot 76 (Figures 3 and 5) is formed vertically through frame members 40 and 42 to provide space for the thread forming rolls 20, 22 which are journalled for rotation on shafts 78. Each of these shafts 78 is supported at one end by a flanged bushing 80 stationarily mounted in the side of the respective frame members 40 and 42 with its outer end surface substantially flush with the surface of the frame member. As shown in Figures 5 and 8, an arcuate groove 82 is formed about the periphery of each bushing 80. A threaded opening 84 is provided in each frame member and extends tangentially to the bushing 80 to enable a cone pointed screw 86 to be inserted in the opening 84 and engage a portion of the circumferential groove 82 to prevent axial movement of the bushing 88. A pin 88 is passed through the flange of each bushing 80 and is embedded in the side of the respective frame members 40 and 42 for the purpose of preventing rotation of the bushings.

In each bushing 80 the cylindrical recess 9t) for receiving and supporting one end of the shaft 78 is formed eccentrically with respect to the outer diameter of the busing. The axis of maximum eccentricity of the respective recesses 90 lies in a plane containing the roll centers and normal to a line joining the roll centers so that, because of this eccentricity, the axis of each shaft 78 is inclined in such a plane only without moving the axes of the two rolls out of the parallel planes containing the axes of eccentricity. In this manner the above described inclination of the axis of each roll is effected to align the threads thereof with the thread on the work piece. Since the rolls 2i and 22 operate upon opposite sides of the work piece the rolls are relatively inclined in opposite directions with respect to the work piece axis. In a typical instance with a /2" work piece on which 13 threads per inch are to be formed with a helix angle 3 7', the roll helix angles will be 3 19 and 3 17' and the eccentricity of bushings 80 will provide an inclination of 0 12' and 0 10 with respect to the roll axes. The bushings 80 are not adjustable and are replaced by others having a different specific eccentricity when it is desired to change the inclination of the axes of shafts 78 as when a work piece of different size or thread characteristics dictates a change in the relative helix angles of rolls and work piece.

Each shaft 78 is also supported at the outer end by a bushing 92, stationarily mounted in the respective frame members and having an outer periphery which is eccentric to the recess 94 occupied by the end of shaft 78. A pair of notches 96 having inclined sides are formed on the periphery of each bushing 92, 180 apart and aligned with the axis of eccentricity of the external andinternal surfaces of the bushing. As shown in Figures 1 and 5, a screw 98 is threadedly engaged in each frame member 40 and 42, with its axis parallel to that of the adjacent bushing 22 and with its head adapted to engageone of the notches 96. Thus each screw 98 serves to retain a bushing 92 in position both rotatively and axially.

The eccentricity of bushings 92 which is normally approximately .0015" is provided to relatively adjust the axes of the shafts 78 so that the working surfaces of the rolls 20 and 22 are parallel despite wear and manufacturing inaccuracies. Ordinarily the elements of the operating surfaces of the rolls are substantially parallel to the roll axis and they will be so considered herein. An indicia such as a line 1% (Figure 1) may be inscribed on the outer surface of each bushing 92 to indicate the relative rotative position of the bushings. This line 100 is, in the present instance, located perpendicular to the 8 axis of eccentricity and on the same side of bushing 92 as the eccentricity. Therefore, the setting shown in Figure 1 indicates that the eccentricities of both bushings are disposed toward the top of the tool. Due to the location of the notches 96, only a 180 shift is possible for each bushing with no intermediate steps. Thus no setting of bushings 92 will introduce any component of inclination of the axes of rolls 20 and 22 into any plane except that passing through the centers of the two notches 96 on each bushing. It will also be noted that this arrangement makes possible four distinct combinations of the relative positions of the eccentricity of the bushings; two in which they are disposed toward the same side of the tool, and two in which they are disposed toward opposite sides of the tool. These settings are considered suflicient to maintain accurate parallelism of the axes of the thread rolls and the work piece within the relatively short thread lengths on which the device is used.

The opposed recesses 90 and 94 are made relatively shallow and the clearances between the shaft 78 and both recesses are such that no binding occurs in either recess as a result of the eccentricity of the other. It will be understood that the total amount of axial inclination in either plane is not large.

Inwardly of the bushings 92, the openings in frame members 40 and 42 through which the shafts 78 pass, are formed with flat sides 102 (Fig. 9) to fit closely similarly flattened portions 104 on the end of each shaft 78. The shafts are thus prevented from rotating relatively to the frame members 40 and 42.

The opening in the respective frame members for each shaft 78, inwardly of the above-described keying engagement and communicating with the slot 76, is enlarged to accommodate a spur gear 1416 which is journalledfor rotation on shaft 78. As shown in Figure 5, each gear 106 has a hub 108 extending axially toward each threading roll 20, 22. Each hub 108 is provided with opposed substantially rectangular axial projections 110 (one shown) which are received within the opposite ends of a 4 suitable slot 112 in the end surface of the respective threading roll. Thus the threading rolls 20, 22 are keyed to rotate integrally with the gears 196 in the form of the invention thus far described.

As shown in Figures 6 and 10, the roll gears 106 are connected for uniform rotation by a series of three spur gears 114, 114, 116, for which suitable recesses 118 are provided in frame members 40 and 42. The central gear 116 is journalled for rotation upon a bushing 120 on the shaft 30 and is held in position axially by a flanged cap member 122 also surrounding shaft 30. The remaining idler gears 114, 114 are rotatively mounted upon bushings 124 on stud shafts 126 which are embedded in the frame members 40 and 42 and protrude outwardly centrally of the recesses 118. Flanged cap members 128 are provided to close the recesses 118 and to retain the idler gears 114, 114 in proper axial position. It will be noted (Figures 1 and 6) that the central cap member 122 overlaps both cap members 128 to retain the latter in position. The cap member 122 is in turn prevented from moving axially by the engagement of the head of a screw 130 (Figure 7) with a suitable notch in the flange of said cap member.

A bushing 1327surrounds each gear hub 198 and is provided with a flange which extends into the slot 76 to form a wear plate for each threading roll. A pin 134, frictionally fitted in each frame member 40, 42 and extending into suitable openings in each bushing 132, prevents rotation of said bushing. On the opposite side of each threading roll 20, 22 and surrounding shaft 78, another wear plate 136 is partially embedded in the side wall of slot 76 to prevent the threading roll from coming into contact with said wall. The pins 88, described above also serve to hold wear plates 136 against rotation. Between them, the bushings 132 and plates 136 closely define the lateral position of threading rolls 20 and 22,

H) but a small amount 'of clearance is provided for purposes of safety and to allow a slight endwise movement of the rolls as they are withdrawn from the work piece after completion of the thread rolling operation.

When the tool reaches the limit of its feed movement and the thread is rolled on the work, the tool usually dwells momentarily before being withdrawn laterally from the work. It will be apparent that compensation of the rolls for the dwell and return movements as well as the feeding movement is impossible, since the threading and withdrawing movements differ in direction and speed. Therefore, a substantial load is abruptly imposed upon the gears when the tool is withdrawn following the threading operation as the rolls tend to slip on the surface of the work. To absorb this sudden force and to minimize the above mentioned endwise movement of the rolls, a resilient connection of novel construction is provided between the threading rolls and the associated gears as shown in Figures 13 and 14.

Each of the modified gears 138, which may replace gears -106, is formed with a hub 139 extending axially toward the respective threading rolls 20 and 22. Each hub 139 is provided with two diametrically opposite projections 140 having flat top and bottom walls 142, the projections 140 being so dimensioned as to fit within slot 112 in the associated rolls with an appreciable clearance. A hole 143 is drilled through the gear and hub parallel with the axis of the hub to intersect each wall 142.

- Positioned within the holes 143 are cylindrical spring wires 144, the free ends of which are flush with the ends of projection 140. The spring holes are enlarged within the projections 146 as at 146 to permit the springs 144 to be deflected toward the center line of the gear. As shown in Figures 13 and 14 the cylindrical surfaces of the springs 144 normally extend beyond the walls 142 and the springs are so spaced that they fit closely within the slots 112 in the end surface of the respective threading rolls, maintaining a clearance between the slots and the walls 142. This position of the springs 144 is maintained during the threading operation since, as explained above, no torque is transmitted from one roll to the other at this time both rolls having the proper tracking relationship with the work piece. When this relationship is disturbed as when the feeding movement of the total is discontinued, the resulting forces implosed upon the driving connection between the rolls and the associated gears are resiliently absorbed by the springs 144. The clearance between the walls 142 and the adjacent sides of the slot 112 is sufiicient to permit the necessary relative angular movement between the thread rolls and the associated gears. When the tool is withdrawn so that the rolls 20 and 22 no longer make contact with stated objects of the invention have been attained by the provision of novel apparatus having cooperating features which produce work pieces of increased accuracy and at the same time reduce the stresses on the thread forming serrations of the threading rolls to a negligible value thus substantially increasing the service life of the thread rolls. Specifically, the stated objects of the invention are accomplished by the provision of threading rolls having different diameters, the diameters of the rolls being such as to eliminate slippage between the rolls and work piece and practically eliminate lateral stresses, by the provision of threading rolls having different helix angles, and by the provision of means for aligning the thread serrations on the rolls with the formed threads on the work piece, all of which features cooperate in a novel manner previously unknown in the art.

This application is a continuation-in-part of application Serial No. 380,558, filed September 16, 1953, and

llll now abandoned, which was in turn a continuation-inpart of application Serial No. 345,362, filed March 30, 1953, and now abandoned.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A device for rolling screw threads by movement laterally onto a work piece comprising, support means; a pair of threading rolls rotatably mounted on said support means, said rolls being of different diameter, one of said rolls having a diameter equal to a normal diameter plus a correction factor, the other of said rolls having a diameter equal to said normal diameter minus said correction factor, said normal diameter being substantially equal to a whole number multiple of the average between the work blank diameter and the minor diameter of the finished work piece and said correction factor being the quotient resulting from dividing pi into the distance of ti e peripheral movement of the point of contact between one of said rolls and the work piece in one revolution of said roll.

2. The thread rolling device defined in claim 1 together with means in said support means for inclining the axes of said rolls in opposite directions with respect to the axis of the Work piece in planes containing the roll centers and normal to a line joining the roll centers to thereby align the threads of both of said rolls with the threads of the work piece.

3. A device for rolling screw threads by lateral movement onto a work piece comprising, a frame; a pair of threading rolls rotatably mounted on said frame, said threading rolls having differing diameters, the difference between said diameters being substantially equal to twice the distance travelled by the point of contact between one of said rolls and the work piece in one revolution of said one roll divided by pi.

4. A device for rolling screw threads by lateral movement onto a work piece comprising, a frame; a pair of threading rolls having helical thread forming serrations, said rolls being of different diameters and having differing helices, means mounting said rolls in said frames for rotation about axes inclined relative to the axis of the work piece in planes containing the roll centers and normal to a line joining the roll centers by an amount equal to the respective differences between said helix angles of said rolls and the helix angle of said work piece whereby said thread forming serrations of said rolls are substantially aligned with the threads formed on the work piece.

5. A device for rolling screw threads of a given pitch diameter by movement laterally onto a work piece comprising, a frame adapted to be laterally moved toward and away from said work piece; a pair of threading rolls having helical serrations, said rolls having different major diameters and different helix angles, the diameter of one roll being greater than a normal roll diameter and the diameter of the other roll being less than said normal diameter, said normal diameter being substantially equal to a whole number multiple of the average between the work blank diameter and the minor diameter of the finished work piece; and means in said frame supporting said threading rolls for rotation about axes inclined with respect to the axis of the work piece in planes containing the roll centers and normal to a line joining the roll centers, the inclination of each roll being equal to the difference between the roll helix angle based on the major diameter thereof and the helix angle 12. of the work piece based on said pitch diameter, such inclination being effective to align the respective contacting portions of the serrations on both rolls with the threads of the work piece.

6. A device for rolling screw threads of a given pitch diameter by movement laterally onto a work piece comprising; a support adapted to be laterally moved toward and away from said work piece; a pair of threading rolls having helical serrations, said rolls having different major diameters and different helix angles, the diameter of one roll being greater than a normal diameter and the diameter of the other roll being less than said normal diameter, said normal diameter being substantially equal to a whole number multiple of the average between the work blank diameter and the minor diameter of the finished work piece; means in said frame members rotatably supporting said threading rolls with their axes inclined with respect to the axis of the work piece in planes containing the roll centers and normal to a line joining the roll centers, the inclination of each roll being equal to the difference between the roll helix angle based on the major diameter thereof and the helix angle of the work piece based on said pitch diameter, said inclination being effective to align the respective contacting portions of the serrations on both rolls with the threads of the work piece; means mounting said frame members on said support for pivotal movement about an axis substantially parallel to the axis of said rolls; and means for adjusting the relative position of said frame members comprising, a strap member secured to one of said frame members and having a portion received in a recess on the other of said frame members; means mounted in said other of said frame members engageable with said portion of said strap member for moving said other frame member with respect to said strap member; and means for locking said other frame member and said strap member in adjusted position.

7. in a device for roll forming work pieces by movement laterally onto the work piece, said device being laterally withdrawn. after a forming operation; a pair of forming rolls having a normal angular relation; a gear train connecting said rolls; said gear train including at least one pair of coaxially mounted relatively rotatable members, means forming a slot in one of said members, a projection formed on the other of said members adapted to be received in said slot in clearance relation thereto, and a plurality of springs mounted on said other of said members adapted to normally contact the sides of said slot in said one of said members to maintain said members in predetermined angular relation, said springs being displaceable to permit a predetermined change in said angular relation in both directions to thereby permit relative angular movement between said rolls when said device is laterally withdrawn and thereafter to restore said rolls to said normal relative position.

3. A resilient connection between a driving member and a driven member comprising means forming a slot in one of said members, a projection formed in the other of said members adapted to be received in said slot in clearance relation thereto, and a plurality of springs mounted on said other of said members adapted to normally contact the sides of said slot in said one of said members to maintain said members in predetermined angular relation, said springs being displaceable to permit a predetermined change in said angular relation in both directions.

References Cited in the file of this patent UNlTED STATES PATENTS 345,959 Fairbairn July 20, 1886 2,257,234 Gould Sept. 30, 1941 2,355,132 Affieck Aug. 8, 1944 (Other references on following page) 13 14 UNITED STATES PATENTS FOREIGN PATENTS Ostendorf May 1, 1951 129,081 Australia Scpt. 8, 1948 Adcock May 22, 1951 510,513 Belgium Apr. 30, 1952 2??? ::::::::::::1 1%, 1321 5 OTHER REFERENCES Erdelyi Oct. 18, 1955 Machinm'ys Handbook, page 1205, 14th edition. 

